1. Field of the Invention
The present invention relates to a thermal head mounted on a thermal transfer printer, for example, and a method for manufacturing the same.
2. Description of the Related Art
A thermal head includes a heat storage layer made of a highly heat-insulating material, a plurality of heating elements which generate heat by energization, a plurality of individual electrodes, each electrically connected individually to one end-portion of a heating element in the length direction of the resistance of the plurality of heating elements, a common electrode electrically connected to the other end-portions of all heating elements, and a protective layer to protect these plurality of heating elements, individual electrodes, and common electrode, on a substrate having excellent heat-dissipating property and made of Si, a ceramic material, or a metallic material, for example. Such a known thermal head performs printing operation by pressing the heating elements against an ink ribbon and a printing substrate wound around a platen roller while the heating elements are generating heat through the common electrode and the individual electrodes.
In recent years, requirements for high output thermal heads capable of performing high-quality printing with higher definition and high-speed printing have been intensified. Therefore, it is indispensable to reduce common drop (drop in voltage of common electrode) and to improve the printing quality. In order to reduce the common drop, the area and the thickness of the common electrode may be increased so as to reduce the common resistance. However, if the common electrode becomes large or thick, achievement of miniaturization becomes difficult. Consequently, up to now, as shown in FIG. 9, an upper common electrode 15 connected to one end-portion of each of a plurality of heating elements 18 is provided and, in addition, a lower common electrode 16 having an area larger than the area of the upper common electrode 15 is provided as a layer located under a resistance layer constituting the plurality of heating elements 18 with an insulating layer 14 therebetween. The upper common electrode 15 and the lower common electrode 16 are electrically connected to each other through a contact hole 14a disposed at a specific position and, thereby, a common electrode is provided. By forming the common electrode having a two-layer structure composed of the upper layer and the lower layer as described above, the common resistance can be significantly reduced, and both the reduction of the common drop and the miniaturization of the head can be satisfied (refer to Japanese Examined Patent Application Publication No. 6-61947, Japanese Unexamined Patent Application Publication No. 10-34991, and U.S. Pat. No. 6,201,558).
However, it was made clear that when the common electrode had a two-layer structure composed of the upper layer and the lower layer as described above, layers in between the upper common electrode and the lower common electrode short-circuited frequently and leakage currents occurred. Since the contact hole is used in order to ensure continuity between the upper common electrode and the lower common electrode, there is a problem in that the manufacturing process is complicated.